Method of reprocessing sewage sludge

ABSTRACT

In a method of reprocessing sewage sludge in which the sludge is dried before it is burnt, it is proposed to use steam from the water-steam circuit of a fossil-fueled power station process as the heating medium for drying and to take the vapors arising during drying into a fire chamber downstream of the power station furnace to reduce nitrogen oxides. The dried sewage sludge is preferably burnt together with coal in the power station furnace.

BACKGROUND OF THE INVENTION

The invention relates to a process for the reprocessing of sewagesludge.

Because of its organic components, the reprocessing of sewage sludge isperformed mostly by combustion whereby the very high water contentproves, however, to be a great impairment to the combustion process.

Thus, it has already been suggested that, prior to its combustion, thesewage sludge be subjected to a mechanical partial dehydration or thatthe sewage sludge be mixed with corresponding volumes of a dry fuel.Both methods have the disadvantage that heat required for theevaporation of the still large volume of residual water is provideddisadvantageously at an unnecessarily high temperature level during thecombustion.

Thus, it has also already been suggested that the sewage sludge besubjected to a thermal drying prior to its combustion. For this reason,atmospheric fluidized bed dryers e.g. have been introduced in which theheat necessary for the drying process is generated in an atmosphericfluidized bed combustor and is added to the fluidized bed dryer via anintermediate heat transfer cycle. The exhaust vapors produced during thedrying process are recycled in part as a carrier medium for thefluidized bed into the fluidized bed dryer and are, for the remainder,condensed outside the installation and are processed correspondingly asan exhaust vapor condensate. Disadvantages of this known process are therelatively high expenditure for process-specific installation parts suchas exhaust vapor purifier, exhaust vapor concentrator, exhaust vaporcondenser, exhaust vapor condensate processor, and generation andtransfer of the required drying heat via pipe walls which are in contactwith the exhaust vapors on both sides.

SUMMARY OF THE INVENTION

The invention is based on the task of developing a simple and economicalprocess for the thermal reprocessing of sewage sludge.

According to the invention this task is solved in that the sewage sludgeis dried first by indirect heat exchange with condensed steam from thewater-steam cycle of a power plant process operated with fossil fuels;that the exhaust vapors produced during drying are passed into acombustion chamber downstream from the furnace zone of the steamgenerator of the power plant process in order to reduce the nitricoxides formed in the furnace zone and that the dried sewage sludge isburned.

The suggestion according to the invention permits both an economicallyand ecologically advantageous reprocessing of sewage sludge throughclose combination with a power plant process operated with fossil, i.e.solid, fluid, or gaseous fuels. In this way the drying heat required forthe sewage sludge may be provided at the required temperature level in asimple manner through low temperature steam, e.g. in the form of drawnoff steam which is essentially already worked off in a steam turbine andstems from the water-steam cycle of the power plant. In the case ofsufficiently large sewage sludge water volumes it is possible to adjustthe necessary temperature level in the dryer by means of the pressure ofthe steam which is used as heating medium, e.g. by way of acounter-pressure turbine or also with a regulated bleeding condensationturbine.

In the case of comparatively low sewage sludge water volumes thenecessary heating steam may be taken from the bleeder at the temperatureand pressure level suitable in each case. For this purpose the supplyingbleeder may also be alternated by switchover.

The exhaust fumes produced during the drying of the sewage sludgecontain, in addition to the water steam, combustible components andother organic compounds, e.g. nitrogen compounds. According to theinvention, these exhaust fumes are fed under precisely definedconditions to a reduction zone which is downstream from the furnace zoneof the fossil power plant process, in order to reduce the nitric oxidesin the flue gas of the fossil power plants, possibly together with otherreducing substances, such as reducing gases or reducing vapors. Theyreact with the nitric oxides which are present in the flue gas, reducingthem to molecular nitrogen in the process. The reduction zone issucceeded by the recombustion with recombustion air of the remainingcombustible materials. The incombustible components of the exhaust fumesfrom the sewage sludge dryer leave the reduction zone together with theflue gas and are fed to the downstream installations of the power plantfor flue gas purification. In this way an individual processing of theexhaust fumes in their own installations is not required according tothe invention.

The infrastructure of the existing power plant process which is operatedwith fossil fuels may also be used advantageously during the combustionof the sewage sludge, if according to another characteristic of theinvention the flue gases produced during the production of the driedsewage sludge are mixed with the flue gases of the fossil power plantprocess and are discharged from the power plant together with them. Inas far as the fly ash produced during the combustion of the sewagesludge is harmless, the sewage sludge flue gas is mixed with the fluegases of the power plant process without prior dust separation so thatthe electric filters of the power plant may also be used for theseparation of dust from the sewage sludge flue gas and the existingheating surfaces of the steam generator may be used for cooling the fluegases.

In the case that the fly ash produced during the combustion of thesewage sludge contains components which do not permit a mixing with thefly ash from the power plant process, another characteristic of theinvention provides a prior separation of this ash. In this case theexisting installations for the desulfurization and denitrogenation ofthe power plant flue gases may also be used for the flue gases from thesewage sludge combustion.

In particular, in cases when the sewage sludge ash or the fly ashproduced during the sewage sludge combustion contains hazardoussubstances, e.g. heavy metals such as cadmium and lead, the sewagesludge may, for improved disposal, be simply melted in its owncombustion chamber with correspondingly high combustion temperaturesand/or in heat exchange with hot flue gas from the power plant process.Ash ceramized in this manner may be used as construction material orfilling material without problems, or may also be discarded. Accordingto another characteristic of the invention, the cooling and melting ofthe ash or fly ash from the sewage sludge combustion may take place atvarious temperature levels which may be adjusted by quenching orcorresponding cooling surfaces. This makes it possible to separate ashcomponents with different melting temperatures.

It is particularly advantageous when according to another characteristicof the invention the dried sewage sludge itself is burned directly inthe furnace zone of a coal power plant. This will especially be the casewhen the furnace zone is operated at temperatures above the meltingpoints of the produced ash components. Hereby the grinding of the driedsewage sludge may be performed according to need either together withthe coal or in its own mill.

In this case the sewage sludge ash is melted completely or in parttogether with the coal-derived ash and is removed as a granulate fromthe power plant and used otherwise. If the power plant process is basedon a dry combustor, i.e. a combustor with temperatures below the ashmelting point, the entire ash is removed together with the flue gas fromthe boiler and is separated in the individual zones of the electricfilter. The known dust separation characteristics of an electric filterhereby result in a certain classification of the fly ash in theindividual zones themselves, depending on the respective particle size.

Since the sewage sludge from the corresponding sewage plants as a ruleis generated continuously, while fossil power plant processes, e.g. hardcoal power plants, are operated only temporarily, i.e. in intermediateor even only in peak load operation, it is useful according to anothercharacteristic of the invention to provide storage possibilities bothfor the fresh raw sewage sludge and/or the dried sewage sludge and/orthen condensed exhaust vapors from the sewage sludge dryer in order toseparate sewage sludge reprocessing and power plant operation.

The invention may be performed in connection with fossil power plantprocesses which are operated with gaseous, liquid, and solid fuels. Butit is particularly advantageous in connection with a power plant processon a hard coal basis.

The main field of application of the invention lies in the reprocessingof sewage sludge. Other solids, however, with a high water content, e.g.river, channel, or harbor sludge may naturally also be treatedanalogously.

Further explanations regarding the invention are found in the exampleshown schematically in the drawing.

BRIEF DESCRIPTION OF THE DRAWING

The drawing is a schematic representation of reprocessing sewage sludge.

DETAILED DESCRIPTION OF THE DRAWING

According to the process shown schematically in the drawing, raw sewagesludge with a water content up to 80% and more is fed via duct 1 firstinto an intermediate storage 2 and then via duct 3 into a dryer and isdried there until a residual water content below 20% is achieved. Thedryer 4 may be a fluidized bed dryer or another known installation whichis operated with steam. The dried sewage sludge is fed to anintermediate storage 5. Sewage sludge from the intermediate storage 5 isthen ground in a mill 6 together with coal which is fed via duct 9, isdried more, and is then fed via duct 7 into the furnace zone 8 of a coalpower plant and is burned there together with coal. The combustion airis fed via duct 10 to the furnace zone 8 which is in this caseconstructed as a dry combustor.

The combustion heat produced in the furnace zone 8 is transferred viaheating surfaces 11 to the water-steam cycle of the power plant withturbine sections 12 and 13, a condenser 14, a feed water pump 15 as wellas preheaters, feed water containers, etc. which are not shown here.

The flue gases 8 leaving the furnace zone 8 reach a reduction zone 17where the nitric oxides are reduced. After this, the flue gases passthrough an air preheater (not shown here) and are then dedusted in anelectric filter 19 with ash output 20.

In installation parts 21 which are shown here only schematically, theflue gas is purified of hazardous substances such as SO₂ and possiblyNOx and is then discharged into the atmosphere via installations notshown here.

The drying heat necessary in the dryer 4 is added via heat exchangersystem 22. For this purpose, drawn off steam which is essentiallyalready worked off is taken from a point downstream from the turbinesection 12 via duct 23, is condensed in the heat exchanger system 22,and is again fed into the water-steam cycle of the power plant via duct24. The steam volume remaining from turbine section 12 is passed viareducing damper 16 to the second turbine section 13. Hereby the pressureof the steam removed via duct 23 may be regulated via the reducingdamper 16.

The exhaust vapors produced in the sewage sludge dryer which in additionto the water steam still contain combustible components are fed via duct25 to the reduction zone 17 which is downstream from the furnace zone 8.In the reduction zone 17 the reducing components of the exhaust vaporstogether with other reducing medium streams which are passed throughduct 18 are reacted with the nitric oxides of the flue gas. In order toburn the remaining combustible materials in the flue gas behind thereduction zone 17, combustion air is added via duct 27. After the fluegas has transferred its heat, it is removed via purificationinstallations 19 and 21 of the power plant.

In order to compensate the exhaust vapor stream 25 and the furnace power8, it is possible to temporarily condense excess exhaust vapors in anexhaust vapor condenser 28, store them in an exhaust vapor condensatecollection container 29, and to feed them again via duct 30 to the dryer4 when the proper furnace power is established so that they areevaporated there.

Dry sewage sludge and exhaust vapor condensate may also be removed fromthe storage 5 or the condensate collection container 29 to be used inother applications outside the power plant.

I claim:
 1. A process for reprocessing sewage sludge using a fossil-fuelpower plant comprising:initially drying the sewage sludge in indirectheat exchange with condensed steam from a steam generator of the powerplant; supplying exhaust vapors produced during the drying process intoa reduction zone of a combustion chamber of the power plant for reducingnitric oxides and other toxic substances; supplying the dried sewagesludge to a furnace zone of the combustion chamber of the power plantfor burning the sewage sludge and thereby producing ash and exhaustgases.
 2. The process of claim 1, further comprising mixing the exhaustgases formed by burning the sewage sludge with flue gases from the powerplant.
 3. The process of claim 2, further comprising passing the exhaustgas and flue gas mixture through a filter for dust-separation.
 4. Theprocess of claim 3, further comprising passing the exhaust gas and fluegas mixture through a purifier.
 5. The process of claim 1, wherein theburning comprises burning the sewage sludge at temperatures above amelting point of the ash.
 6. The process of claim 1, further comprisingmelting the ash produced during the burning of the sewage sludge usingheat from hot flue gases of the power plant.
 7. The process of claim 6,wherein the melting comprises melting the ash at different temperaturelevels.
 8. The process of claim 1, further comprising grinding thesewage sludge together with the fossil-fuel of the power plant forsupplying to the furnace zone of the power plant.
 9. The process ofclaim 1, further comprising grinding the sewage sludge before supplyingto the furnace zone of the combustion chamber of the power plant. 10.The process of claim 1, further comprising separately storing the sewagesludge and the exhaust vapors, both before and after drying, each in anintermediate storage means respectively.